Phenol and methylene blue adsorption on heat-treated activated carbon: Characterization,kinetics,and equilibrium studies

Abstract

A comprehensive study was performed for a thermally treated activated carbon to evaluate the influence of this treatment on the physical and chemical properties of the mineral activated carbon, as well as the adsorption toward phenol and methylene blue. After the heat treatment, surface area decreased and total pore volume diminished about 8.5%, and the total basic groups decreased 18% while the total acid groups increased 8% in comparison with the raw activated carbon. Equilibrium adsorption of phenol and methylene blue was described well with the Freundlich and Langmuir isotherm models, respectively. Adsorption kinetics of phenol and methylene blue was predicted adequately with the empirical pseudo-second-order model, the intraparticle diffusion model, and the homogeneous solid diffusion model, but mass transfer coefficients of the diffusion models help to better understand the adsorption phenomenon. Intraparticle diffusion seems to be the rate-controlling step in the adsorption process, and heat-treated activated carbon in an inert atmosphere was a better adsorbent for both phenol and methylene blue than raw activated carbon.

Keywords

Water treatment adsorbent surface chemistry textural properties modeling intraparticle diffusion

Introduction

Phenol is among the topmost toxic chemicals and causes an unpleasant taste and odor when present in water. If drinking water contains phenol, it may cause severe renal insufficiency, convulsions, and even death. Phenolic compounds are contained in wastewater from coal distillation, organic synthesis, pulp and paper bleaching facilities, resin, pesticide, insecticide, paint, and solvent industries. Chlorination of wastewater containing phenol for disinfection purposes may cause the formation of chlorophenols, which usually are corrosive to skin and eyes. Phenolic compounds can be removed by chemical or biological methods; however, the removal of phenol by adsorption on activated carbon is the most frequently used method in comparison with other processes such as aerobic and anaerobic biodegradation, oxidation with ozone, and the usage of ion exchange resins (Gundogdu et al., 2012).

Similarly, dyes are usually contained in effluents from industries such as textiles, printing, pulp mills, leather, food, dyestuffs, and plastics, and most of these dyes are resistant to biodegradation because of its complex and stable molecular structure. When these dyes containing effluents are discharged in water bodies, they reduce sunlight transmission affecting photosynthetic activity of organisms and, consequently, the development of aerobic forms of life decreases due to a reduction of the dissolved oxygen in water. Dyes can be classified as anionic, cationic, and nonionic, and a number of technologies including biological treatment, coagulation/flocculation, ozone treatment, chemical oxidation, membrane filtration, ion exchange, photocatalysis, and adsorption have been developed for the treatment of dyes containing effluents (Ai et al., 2011; Oladoja and Akinlabi, 2009). A cationic dye (3,7-bis(dimethylamino)-5λ⁴-phenothiazin-5-ylium chloride, commonly known as methylene blue) was selected in this research. Methylene blue is commonly used in the staining of cotton, wood, and silk and it may cause allergic reactions in eyes, vomiting, dizziness, cyanosis, and an increment in blood pressure when people drink water containing this pollutant (Sharma et al., 2010). The usage of phenol and methylene blue will provide information about the role of surface functional groups in adsorption of both ionized and not ionized molecules, as well as the importance of the molecular size in the adsorption process.

Activated carbon has proven to be a suitable adsorbent for a wide variety of organic compounds present in water. It can be produced from several carbonaceous precursors including wood, coal, coconut shells, and sugar cane bagasse, among others. The structure of activated carbon consists of carbon atoms ordered in parallel piles of hexagonal layers linked tetrahedrally, and the adsorption properties of this material are mainly attributed to the surface chemistry (i.e. functional groups), pore volume, and specific surface area. Functional groups on the activated carbon surface contain predominantly oxygen, but they may also contain hydrogen, nitrogen, and sulfur. The activated carbon surface can be acid, basic, or neutral depending on the type and quantity of its functional groups (Green and Martin, 1968; Shaarani and Hameed, 2011). The basic character of activated carbon is associated with pyrone and chromene functional groups, as well as oxygen-free Lewis base sites on the basal planes; whereas carboxylic, carbonyl, phenols, enols, lactones, and quinones are related to acid functional groups. In order to modify the surface chemistry of activated carbon, different treatments can be carried out. Most of the treatments reported in literature consist of heating the activated carbon in presence of different gases, as well as oxidation, amination, and impregnation with different chemicals. These treatments may change not only the chemical and structural characteristics of the carbon but also the surface reactivity toward specific compounds of interest, such as ionized or neutral organic compounds. It has been reported that when activated carbon is treated at high temperatures under nitrogen atmosphere, the oxygen-containing sites are removed and the carbon basicity increases, which may enhance the adsorption capacity for phenolic compounds (Zhang et al., 2016), but this surface is not stable, and can readsorb oxygen even at room temperature and rapidly become reacidified. This phenomenon has been attributed to a gradual surface oxidation, and it may occur faster if this heat-treated carbon is stored in a moisturized environment, being activated carbon with basic properties and nitrogen-containing groups more resistant to this oxidative effect. Menéndez et al. (1996) proposed a model for explaining these reactions considering, in particular, the pyrone and lactone groups (they assumed all the other acidic oxygen surface groups suffer a similar fate). This model establishes that after the activated carbon is thermally treated in a nitrogen atmosphere, isolated unpaired electrons (free radicals) edge sites are formed, as well as carbon atoms with a triple bond character, leaving highly reactive carbon atoms in the resultant surface (Green and Martin, 1968, Menéndez et al., 1996).

Additionally to the physical and chemical properties of the adsorbents and the adsorbate, equilibrium studies allow to select an appropriate adsorbent for a given adsorbate in a specific set of conditions (i.e. pH, temperature, and biomass dosage). Similarly, kinetic experiments help to choose the adsorbent that have the fastest adsorbate uptake rate and are useful to determine the rate-limiting step in the adsorption process as well as to design packed bed adsorbers.

Most of the authors use empiric models to describe the kinetic adsorption process, for example the pseudo-second-order (PSO) model (Ho and McKay, 2000), but this type of expressions only help to obtain a global kinetic constant and, consequently, a more detailed explanation of the adsorption phenomenon and the effect on every stage of it is not possible. Another model commonly used for predicting adsorption kinetics is the intraparticle model (IPM) proposed by Weber and Morris (1963), which recognizes the diffusion coefficient as an independent term, but taking into account only the first minutes of the adsorption process, making it impossible to predict the behavior of the whole process with the information given by this model.

For this reason, a phenomenological model, the homogeneous solid diffusion model (HSDM), is proposed and used in this research (Crank, 1956). By applying the solution proposed by Cranck, a global diffusion coefficient can be determined from the HSDM model. Besides, it is important to mention that this model considers all of the time when the adsorption process is carried out and, therefore, it should offer also an accurate prediction of the process behavior when replicated in similar conditions. The global diffusion coefficient from the HSDM model considers the following stages (Crank, 1956): (i) External mass transfer resistance: film diffusion of solutes from bulk solution through the boundary layer to the surface of the adsorbent, (ii) intraparticle mass transfer resistance: intraparticle diffusion of solute through the adsorbent solid matrix (pore volume diffusion or surface diffusion), and (iii) adsorption of solute molecules at the active sites (commonly the fastest step). To the best of our knowledge, there are no reports of the use of the HSDM model to evaluate the effect of a heat treatment on the activated carbon and their adsorption kinetic parameters for organic molecules present in aqueous solutions.

In this work, a heat treatment under inert atmosphere for a commercial bituminous granular activated carbon (GAC) was performed in order to study the effect of this treatment on its surface chemistry, physical structure, and the adsorption capacity of this adsorbent toward phenol and methylene blue present in aqueous solutions. Mass transfer parameters were also calculated by empirical and diffusion models to assess if the heat treatment of the activated carbon affects the adsorption rate of the selected pollutants.

Methods

Adsorbates and adsorbent

Phenol and methylene blue were selected as adsorbates for this study. These analytical grade reagents were used as received. The commercial bituminous GAC was kindly provided by Clarimex™ and it was used as the starting material to investigate the effect of a heat treatment under inert atmosphere on its surface. It is important to mention that to the best of our knowledge, there are not published reports about adsorption studies on the selected activated carbon (either raw or treated).

Heat treatment

A carbon sample was placed in a quartz reactor with a constant nitrogen gas flow. The sample was heated at a rate of 10℃/min until 800℃, and then the heating rate was reduced at 5℃/min until a temperature of 900℃ was reached. The carbon was kept at 900℃ for 2 h, and subsequently it was allowed to cool down at room temperature maintaining the nitrogen gas flow rate constant. Finally, the heat-treated carbon was sieved to a size of 1–2 mm, stored in a desiccator for future use and identified as heat-treated activated carbon (HGAC) throughout the document.

Physical surface characterization

Specific surface area, total pore volume, and size distribution of the activated carbon before and after the heat treatment were determined from physical adsorption measurements with N₂ at 77 K on samples which were previously degasified at 363 K. The Autosorb 1 automated gas sorption system (Quantachrome Instruments) was used and the surface areas and pore size distribution were obtained with the Brunauer-Emmett-Teller and the Barrett-Joyner-Halenda analysis, respectively. Additionally, surface morphology of activated carbon was analyzed before and after the heat treatment with a scanning electron microscope Jeol JSM-6510LV. X-ray diffraction patterns of raw and heat-treated activated carbon were obtained, with a Siemens X-ray diffractometer, scanning from 5 to 90 2θ° in order to evaluate any possible structural change occurred during the thermal modification.

Elemental analysis

Elemental analysis was carried out in a Perkin Elmer 2400 instrument to determine the carbon, nitrogen, hydrogen, and sulfur content percentage. The oxygen content was determined by difference of these results and the ones for moisture and inorganic compounds content obtained with thermogravimetric analysis.

Chemical functional groups

In order to evaluate the effect of the heat treatment on the surface chemistry of the activated carbon, total basic and acid sites were determined before and after the heat treatment by Boehm titrations (Boehm, 1994; Goertzen et al., 2010) as follows: 250 mg of the carbon was added to 25 ml of NaOH or HCl 0.1 N and the suspensions were stirred with an orbital shaker at 150 r/min and a constant temperature of 25℃ for one week. After that time, a 10 ml aliquot was titrated with standardized NaOH or HCl 0.1 N and the quantity of basic and acid groups was calculated. Similarly, Fourier transform infrared (Spectrophotometer Shimadzu IRAffinity-1) was recorded from a wavenumber of 4000 to 400 cm⁻¹ to identify functional groups on the raw and heat-treated activated carbons, before and after the adsorption process of phenol and methylene blue.

Adsorption isotherms

Adsorption experiments for phenol and methylene blue were carried out in duplicate as follows. First, solutions with initial concentration ranging from 10 to 200 mg/l were prepared by dissolving each adsorbate in distilled water and the pH of the solution was subsequently adjusted to 7 with NaOH or HCl 0.1 N as required. After pH was adjusted, 40 ml of solution was placed in 50 ml plastic centrifuge tubes and 40 mg of activated carbon was added at each solution (adsorbent dosage of 1 g/l). All suspensions were continuously stirred at 150 r/min with an orbital shaker and the temperature was kept constant at 25℃ for seven days in the case of phenol, and 15 days for methylene blue, in order to ensure that adsorption equilibrium was reached. The initial and final concentrations were determined by a Thermo Scientific Genesys 10S spectrophotometer, at a wavelength of 270 and 666 nm for phenol and methylene blue, respectively. Adsorption capacity was calculated by equation (1), where q is the adsorption capacity, C₀ is the initial adsorbate concentration in the solution, $C_{e}$ is the adsorbate concentration in solution at equilibrium, V is the volume of the solution, and m is the adsorbent mass quantity.

q = \frac{V (C_{0} - C_{e})}{m}

(1)

Adsorption experiments at equilibrium allow relating the concentration of adsorbate that remains in the solution and the adsorption capacity of the adsorbent, in other words, the quantity of adsorbate removed per unit mass of adsorbent. In this work, Langmuir and Freundlich isotherm models were used to describe the adsorption at equilibrium of phenol and methylene blue with raw (GAC) and heat treated (HGAC) activated carbons. Langmuir model (Langmuir, 1918) considers that the adsorption process is carried out in a monolayer, where all of the adsorption sites are of the same type and they are all occupied at the end of the process. On the other hand, the Freundlich model (Freundlich, 1906) considers that active sites for adsorption are heterogeneous, and there is also a possibility that multilayers of adsorbate may be formed on the adsorbent surface. Langmuir and Freundlich models are represented by equations (2) and (3), respectively

q_{e} = \frac{q_{\max} {bC}_{e}}{1 + {bC}_{e}}

(2)

q_{e} = K_{F} C_{e}^{1 / n}

(3)

where q_e is the adsorption capacity at equilibrium, C_e is the concentration of the adsorbate in the solution at equilibrium,

q_{\max}

is the maximum adsorption capacity, b is the Langmuir constant, K_F is the Freundlich constant, and n is the heterogeneity factor.

Adsorption kinetics

Adsorption kinetic tests were conducted in a differential column batch reactor with a 2 cm inner diameter and a packed bed height of 0.6 cm at room temperature (∼25℃) with 1 l of solution and 1 g of activated carbon, maintaining the dosage of 1 g/l used in equilibrium experiments (Figure A1 shown in supplementary material). Initial pH was adjusted at 7 with HCl and NaOH 0.1 N as required, and the solution was recirculated at a volumetric flow of 400 cm³/min. Aliquots of 1 ml for phenol and 0.5 ml for methylene blue were taken (32 samples) at different intervals of time until equilibrium was reached and the adsorbate concentrations were determined spectrophotometrically.

The obtained adsorption kinetic data were used to obtain the parameters of the PSO model. Also, the effective diffusion coefficient was calculated by using the intraparticle diffusion model (IDM) proposed by Weber and Morris and the HSDM with the mathematical solution proposed by Crank.

The PSO was proposed by Ho and McKay (2000) as shown in the following equation

\frac{dq}{dt} = k_{2} (q_{e} - q)^{2}

(4)

where q_e and q (mg/g) are the adsorption capacities at equilibrium and time t (min), respectively, and k₂ (g/mg min) is the rate constant of the adsorption process.

Integrating equation (4) from $t = 0$ to $t = t$ and from $q = 0$ to $q = q$ , and reaccommodating terms, equation (5) is obtained

\frac{t}{q} = \frac{1}{k_{2} q_{e}^{2}} + \frac{1}{q_{e}} t

(5)

The parameters k₂ and q_e in this model can be calculated by plotting $t / q$ versus t. Moreover, with the estimated values of k₂ and q_e, the initial adsorption rate (h) can be obtained with the following equation

h = k_{2} q_{e}^{2}

(6)

Here h (mg/g min) represents the quantity of adsorbate adsorbed per unit of mass of the adsorbent and per unit of time, and it can be used as a comparative parameter for kinetic adsorption experiments conducted at similar conditions.

Intraparticle diffusion

In order to evaluate the influence of the heat treatment of activated carbon in the diffusion of phenol and methylene blue through this porous material, the effective diffusion coefficient was calculated with two models. In the model proposed by Weber and Morris (1963) (equation (7)), q_t represents the concentration of the adsorbate in the activated carbon at time t; k_p is associated with the intraparticle diffusion rate constant, which corresponds to the slope of the plot; and the intersection I is related with the thickness of the boundary layer

q_{t} = k_{p} t^{0.5} + I

(7)

After obtaining the k_p value, the effective diffusion coefficient D_i can be determined with equation (8), where d_p is the particle diameter

k_{p} = (\frac{3 q_{e}}{d_{p}}) \sqrt{\frac{D_{i}}{π}}

(8)

In the HSDM and its solution first proposed by Crank (1956) (equation (9)), $\bar{q}$ and $q_{\infty}$ are the average concentration of solute in the solid at any time and at infinite time, respectively; t represents the time; R is the particle radius; and D_s is the effective diffusion coefficient.

\frac{\bar{q}}{q_{\infty}} = 1 - \frac{6}{π^{2}} \sum_{n = 1}^{\infty} \frac{1}{n^{2}} \exp (\frac{- D_{s} n^{2} π^{2} t}{R^{2}})

(9)

The value of D_s was obtained with Microsoft Excel™. The sum term in equation (9) was programmed with Excel VBA (Visual Basic for Applications) and the adding terms with a contribution higher than 1 × 10⁻⁶ were taken into account. The variance was chosen as the error function when comparing the experimental and predicted data of $\bar{q} / q_{\infty}$ ; D_s value was changed by using the Solver tool included in Excel™ in order to minimize the sum of the variance.

It is noteworthy that equation (9) is valid only by assuming an “infinite bath” situation, which occurs if the adsorbent is a solid sphere initially solute free and its surface concentration remains constant during the adsorption process (Crank, 1956).

External mass transfer coefficient

The mass transfer coefficient was determined by using equation (10), where C represents the concentration at any time, C₀ is the initial concentration, L is the length of the packed bed, $ε$ is the packed bed porosity, v is the interstitial axial velocity, k_f is the global mass transfer coefficient in the liquid phase, and S₀ is the surface area of the adsorbent particle per unit of particle volume

\frac{C}{C_{0}} = \exp [- \frac{(1 - ε) k_{f} S_{0} L}{ε v}]

(10)

If the values obtained of $C / C_{0}$ are plotted versus time for the first minutes of the process, it can be possible to extrapolate the data to the time when the first fraction of the fluid that entered the column goes out of it, that is when $t = L / v$ , and the value of $C / C_{0}$ can be determined. By using this value, k_f can be calculated if the rest of the parameters are known values (Crank, 1956).

Results and discussion

Heat treatment

Table 1 shows the textural and chemical properties of GAC and HGAC. It can be noted that textural features of the GAC after heat treatment changed, for instance, surface area diminished 8%, pore size distribution was reduced in 10% for micropores, 7.9% for mesopores, and 11% for macropores, as well as total pore volume decreased 8.5% (Table 1).

Table 1.

Textural and chemical properties of the adsorbents.

Property	GAC	HGAC
Surface area (m²/g)	676.7	619.2
Micropore volume (cm³/g)	0.0490	0.0441
Mesopore volume (cc/g)	0.716	0.659
Macropore volume (cc/g)	0.136	0.121
Packed bed density (g/cc)	0.5294	0.5167
Packed bed porosity (unitless)	0.64	0.66
Total acid sites (meq/g)	0.65	0.71
Total basic sites (meq/g)	0.44	0.36
Carbon (%)	70.60	71.90
Hydrogen (%)	0.73	0.74
Nitrogen (%)	0.60	0.54
Sulfur (%)	3.14	3.33
Oxygen (%)	14.43	13.99
Moisture (%)	4.00	4.50
Inorganic compounds (%)	10.50	9.50

GAC: granular activated carbon; HGAC: heat-treated activated carbon.

Similar results were obtained by Menéndez et al. (1996) who reported a diminution of the total weight of wood-based activated carbon of 8.3% after a heat treatment with N₂ at 950℃, and this treatment did not cause any drastic changes in the physical properties of the carbon. They reported that the percentage of oxygen diminished from 15.1 to 4.6% based on elemental analysis, while in this study, the diminution was only from 14.43 to 13.99% (Table 1). It is important to mention that this difference may be attributed to the precursor nature, which may affect the type of oxygen-containing groups (phenolic, carboxylic, lactone, quinone, etc.) on the surface, and oxygen percentage after a heat treatment may vary depending on the quantity of the functional groups that get volatilized more easily during the heat treatment.

Regarding the surface chemistry of the heat-treated activated carbon, there was an increment in the total acid sites of 8%, whereas the basic sites decreased about 18% in comparison with the raw activated carbon (Table 1). Besides, the mineral-based activated carbon used in this work is predominantly meso and macroporous (Table 1); therefore, both phenol and methylene blue should be able to move inside the porous particle during the diffusion step in the adsorption process.

FTIR spectra (Figure A2 shown in supplementary material) showed the characteristic peaks for common functional groups of activated carbon, such as C–C, C–O, C–N, C–H, and O–H, and it can also be noted that the noise to the signal appears after the adsorption process, which may be due to the presence of phenol and methylene blue on the surface, although the characteristic signals for these compounds (C–C, C–O, C–H, O–H for phenol; C–C, C–H, C–N for methylene blue) probably became covered with the ones of the activated carbon and, therefore, it is impossible to note them alone in the spectra.

SEM images (Figure A3 shown in supplementary material) showed that after the heat treatment, the carbon surface became less rough and several pores may get blocked. These results agree with surface area reduction obtained by nitrogen physisorption and with the proposed adsorption mechanism that includes π–π interactions with the carbon surface, which enhances both phenol and methylene blue adsorption by leaving a more available external surface area. Besides, these structural changes could lead to the decrease of the diffusivity constant and the increment in the mass transfer coefficient obtained with the HSDM. Additionally, an EDAX study was performed (Figure A3 shown in supplementary material) revealing that amorphous carbon is mainly present in the selected adsorbents. Aluminum and silicon components were also detected, in lower proportion than carbon, and it can be explained taking into account that a mineral activated carbon was used in this study. Aluminosilicates are usually present in the natural sources as coal materials, and the presence of Si on the activated carbon surface was corroborated with X-ray diffraction (Figure A4 shown in supplementary material).

Equilibrium adsorption tests

After the heat treatment of the activated carbon, the adsorption capacity of phenol and methylene blue increased 24 and 33%, respectively (Table 2). This behavior can be associated mainly with the decrease of basic sites after the heat treatment (Table 1), considering that the pore size distribution was not affected in a higher extent (Table 1). Experimental data obtained from the adsorption isotherms of phenol and methylene blue were fitted well with both Langmuir and Freundlich models, as shown in Table 2. However, by considering the tendency of experimental results (Figure 1), Freundlich model was better for predicting the adsorption of phenol and Langmuir model for predicting methylene blue adsorption at equilibrium.

Table 2.

Langmuir and Freundlich parameters for phenol and methylene blue with GAC and HGAC at 25℃ and pH 7.

Isotherm/ parameters	GAC		HGAC
Isotherm/ parameters	Phenol	Methylene blue	Phenol	Methylene blue
Langmuir
q_max (mg/g)	157.91	130.88	197.11	175.02
b (l/mg)	0.38	6.90	0.57	0.25
R²	0.98	0.78	0.98	0.81
Freundlich
n	3.03	5.60	2.62	3.48
K	48.57	86.44	68.01	57.49
R²	0.98	0.80	0.96	0.78

GAC: granular activated carbon; HGAC: heat-treated activated carbon.

Figure 1.

Adsorption isotherms of phenol (a) and methylene blue (b) in raw (GAC) and heat-treated activated carbon (HGAC) at 25℃ and pH 7. Symbols represent the experimental data and the isotherm model (Freundlich for phenol, Langmuir for methylene blue) is shown as a line.

Other authors have also reported that the adsorption of phenol is well described by both Langmuir and Freundlich models (Alvarez Rodriguez et al., 2011; Din et al., 2009; El-Naas et al., 2010; Giraldo and Moreno-Piraján, 2014; Guocheng et al., 2011; Hameed and Rahman, 2008; Miao et al., 2013; Nabais et al., 2009; Yang et al., 2014) (Table SI shown in supplementary material). In 2012, Gundogdu et al. (2012) studied the adsorption of phenol with activated carbon produced from tea industry wastes and concluded that the Langmuir model described the process well according to the tendency of the curves and the fitting of the data (visually). Miao et al. (2013) remarked that the best model to describe the adsorption of phenol on activated carbon prepared from soybean straw was Freundlich based on the determination coefficient values (R²). Din et al. (2009) proposed that there was a possibility of mono and heterolayer phenol formation on the adsorbent surface due to the surface chemistry of the activated carbon, since active functional groups with high energy level tend to form heterolayers, while active sites with lower energy level will induce monolayer coverage due to electrostatic forces.

The interaction between phenol molecules and activated carbon during adsorption has been explained by different mechanisms (Alvares Rodrigues et al., 2011; Nabais et al., 2009; Yang et al., 2014), for instance: the π–π interactions between the π electrons in the aromatic rings of the phenols and those in the graphene layers, the electron donor–acceptor complex mechanism, and the solvent effect by hydrogen bonds formation. In this work, due to the small quantity of surface functional groups and the pH of the solution, the process is most likely to occur due to the π–π interactions mechanism. Besides, there could be π–π interactions between the phenol species in the solution and the phenol molecules adsorbed on the carbon surface, which may allow multilayer phenol adsorption once the first adsorption layer on the activated carbon surface is formed.

In contrast, for methylene blue, a positively charged molecule in aqueous solutions at the pH value, the predominant adsorption mechanism is probably electrostatic attraction between methylene blue species and the dissociated acid functional groups of the activated carbon (Table 1), making it difficult to form more than one layer when all the acid sites are occupied, and this statement agrees with the fact that the Langmuir model is the one that better describe the nature of the process. Vargas et al. (2011) presented some mechanisms that may occur during methylene blue adsorption on activated carbon, including electrostatic interactions, hydrogen bonding formation, electron donor–acceptor, and π–π dispersion interaction, which are similar to those interactions mentioned for phenol. This means that the predominant mechanism in each particular case (not only for phenol and methylene blue, but for the adsorption processes in general) will be determined by the nature of the adsorbent and the adsorbate, as well as the system conditions (temperature, pH, adsorbate concentration, etc.), without forgetting, nonetheless, that adsorption is usually carried out by a combination of different mechanisms acting simultaneously.

Additionally, it is important to note that for methylene blue adsorption, activated carbon manufactured from biomass precursors showed a better uptake performance than the adsorbent used in this work (Ahmed and Dhedan, 2012; Cazetta et al., 2011; Dural et al., 2011; Foo and Hameed, 2011a, 2011b, 2012; Hameed et al., 2007a, 2007b; Li et al., 2013; Olorundare et al., 2014; Pezoti et al., 2014; Vargas et al., 2011) (see Table SII in supplementary material), a bituminous coal-based activated carbon (i.e. mineral origin precursor). This issue is probably due to the fact that usually activated carbons derived from biomass are predominantly mesoporous adsorbents, making the intraparticle diffusion process to be fast and, therefore, more adsorption sites inside the adsorbent particles are available to adsorb organic molecules, while mineral coal-based activated carbons are usually micro and mesoporous materials, and the diffusion step is slower than for vegetal-based activated carbons.

Adsorption kinetics

The PSO model, the IDM, and the homogenous solid diffusion model fitted well the adsorption kinetic data of both phenol and methylene blue, as shown in Table 3. The initial adsorption rate

(h

) indicates that at the begin of the process, phenol adsorption was faster in GAC (3.21 mg/g min) than in HGAC (2.73 mg/g min), but an opposite effect was observed for the adsorption of methylene blue achieving 0.15 and 0.26 mg/g min for GAC and HGAC, respectively. This may be due to the decrement of basic sites and the volatilization of some of the surface groups that provoked more acid surface in HGAC (Table 1), causing phenol to be repelled by the carbon surface because of the electrostatic repulsion in comparison with GAC, but being favorable for the adsorption of a positively charged molecule as the methylene blue. Additionally, it is important to mention that the initial adsorption rate of methylene blue on HGAC and GAC is considerably lower than those values for phenol adsorption due to the size of the molecule.

Table 3.

Kinetic parameters for phenol and methylene blue adsorption with GAC and HGAC at room temperature and an initial concentration of 200 mg/l.

Model/ parameters	GAC		HGAC
Model/ parameters	Phenol	Methylene blue	Phenol	Methylene blue
Pseudo second order
k₂ (g/mg min)	2.16E-4	1.11E-5	1.66E-4	3.20E-5
q_e (mg/g)	121.95	121.95	128.20	91.74
h (mg/g min)	3.21	0.16	2.73	0.26
R²	0.99	0.96	0.99	0.98
Error	3.95	40.37	9.46	53.35
Intraparticle diffusion
k_p (g/mg min)	9.4179	1.7485	8.6868	1.9795
D_i (cm²/min)	5.10 E-3	2.28E-3	4.40E-3	1.94E-3
R²	0.9198	0.9459	0.9870	0.9435
Error^a	11.83	16.03	7.63	7.22
HSDM
k_f (cm/min)	6.44E-4	3.11E-5	7.23E-4	2.77E-4
D_s (cm²/min)	4.34E-6	1.70E-7	3.54E-6	2.80E-7
Error	17.45	31.66	13.00	7.87

GAC: granular activated carbon; HGAC: heat-treated activated carbon; HSDM: homogeneous solid diffusion model.

In all the cases, error was calculated as: $\frac{1}{n} \sum {no}_{1}^{n} (q_{experimental} - q_{calculated})^{2}$ .

For intraparticle diffusion model, only data of the first slope were considered.

When comparing the performance of the activated carbon used in this study with those derived from biomass, it can be noted that in general adsorption of phenol (Table SIII and SIV shown in supplementary material) and methylene blue follows a PSO model for mineral- or vegetal-based activated carbon (Ahmed and Dhedan, 2012; Alvarez Rodriguez et al., 2011; Cazetta et al., 2011; Din et al., 2009; Dural et al., 2011; El-Naas et al., 2010; Foo and Hameed, 2012; Gundogdu et al., 2012; Guocheng et al., 2011; Hameed and Rahman, 2008; Hameed et al., 2007a, 2007b; Li et al., 2013; Pezoti et al., 2014; Yang et al., 2014); however, the velocity rate constant has higher values for the vegetal-based adsorbents (Table SIII and SIV shown in supplementary material). This behavior can be attributed to the activated carbon properties as discussed previously. Moreover, the initial adsorption rate is similar for phenol adsorption in all the cases, while for methylene blue is lower than the ones reported by other authors with vegetal-based carbons. This can be due to the pore size distribution and the molecular size of the adsorbates, being easier for phenol to go through the activated carbon pores in comparison with methylene blue. The rate parameter value $(k_{p})$ obtained with the Weber and Morris IPM is also in accordance with this phenomenon.

The effective diffusion coefficient (D_s) and the external mass transfer coefficient (k_f) were determined for the adsorption of phenol and methylene blue with GAC and HGAC (Table 3). After the heat treatment and for phenol adsorption, k_f increased from 6.44E-4 to 7.23E-4 cm/min, while D_s decreased from 4.34E-6 to 3.54E-6 cm²/min. In the case of methylene blue, k_f increased from 3.11E-5 to 2.77E-4 cm/min, and D_s changed from 1.70E-7 to 2.80E-7 cm²/min (Table 3). These results suggest that the structural and chemical changes after the heat treatment of the coal-based activated carbon affected the adsorption process of the two selected adsorbates. Similarly, diffusion on the raw or heat-treated activated carbon was slower for methylene blue than phenol according to the value of the effective diffusion coefficient (Table 3).

Among all the existing models (i.e. empirical and diffusion) to describe the adsorption kinetics of pollutants onto any adsorbent, empirical models are the most often used because these are easily applied, but the global kinetic constant do not provide a more detailed explanation of the adsorption phenomenon. In contrast, diffusion models are not commonly used in part because these models require more effort in solving the differential equations. Nevertheless, diffusion models are helpful to understand the adsorbate mass transfer to the adsorbent particle contrary to empirical models. Diffusion models such as HSDM can adequately predict the concentration decay data of phenol and methylene blue on the raw and heat-treated activated carbon (Figure 2) and also helps to explain physically the mass transfer process occurring in the adsorption process.

Figure 2.

Adsorption kinetics of phenol (a) and methylene blue (b) in raw (GAC) and heat-treated activated carbon (HGAC) at room temperature, initial concentration of 200 mg/l, flow rate of 400 cm³/min, and pH 7. The experimental data are represented by symbols and lines represent data obtained by the homogeneous solid diffusion model.

This conclusion is based on various facts: first, the low deviation between the experimental and predicted capacities indicated that this model fits adequately the concentration decay curve (Table 3, Figure 2). Second, the physical properties (i.e. high porosity, surface area, and pore volume) of these adsorbent (Table 1) suggested that the contribution of intraparticle diffusion is higher than the film diffusion. Finally, and not less important, the effective diffusion coefficient (D_s) of the HSDM model has a physical meaning that helps to explain the diffusion of solutes throughout the activated carbon and could be used for predicting adsorption processes in continuous systems.

Conclusions

Specific surface area and total pore volume of the GAC decrease about 8.5% after heating the adsorbent under inert atmosphere at 900℃ for 2 h. Furthermore, the basic total sites decrease 18% whereas total acid sites increase 8% in comparison with the raw activated carbon. After the heat treatment, the adsorption capacity of phenol and methylene blue increases 24 and 33%, respectively. Oxygen and nitrogen quantity diminish slightly due to the volatilization of some organic groups present on the activated carbon surface. Phenol adsorption onto GAC and HGAC follows the Freundlich isotherm model, indicating that adsorption is carried out in multilayers taking into account that this adsorbate is not dissociated at neutral pH.

In contrast, the adsorption of methylene blue, a positively charged molecule, onto GAC and HGAC is described well by the Langmuir isotherm model, suggesting that adsorption occurs in such a particular site as the acid functional groups. On the other hand, although empirical and diffusion models can predict the adsorption kinetics of phenol and methylene blue onto GAC and HGAC, diffusion models such as HSDM can also help to explain the mass transfer process occurring in the adsorption phenomenon, for instance, intraparticle diffusion of the adsorbates inside the porous adsorbent is established as the rate-limiting step. In summary, heat treatment affects the physical and chemical structure of the activated carbon increasing the adsorbate capacity and the mass transfer rate.

Footnotes

Acknowledgements

The authors would like to thank the Universidad Autónoma de Nuevo Léon and the Facultad de Ciencias Químicas for the financial support and infrastructure in the development of this research.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Carina A. Sáenz-Alanís thanks to CONACyT for graduate funding (grant number 237641).

Supplementary Material

The online supplementary materials are available at .

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